1. Field of the Invention
This invention relates generally to the field of electrophotography, and more particularly to means for providing interframe flashdown while illuminating and exposing reflection originals.
2. Description of the Prior Art
It is well known in electrophotography that a latent electrostatic image is formed on a photoconductor by exposing the photoconductor to light reflected from an original document. Frequently the document is illuminated through a transparent supporting platen and the resulting image is focused on the surface of the photoconductor through appropriate optics. In the case of a typewritten document, for example, the background is white, and reflects light through the optics to the photoconductor, while the typing is black, and absorbs light with very little reflection. The photoconductor then "sees", i.e., has projected onto its surface, a pattern or image-wise distribution of light and dark areas representing the background and typing, respectively, of the original document.
The photoconductor is uniformly charged electrostatically prior to its exposure, and the latent electrostatic image is produced by selectively discharging the photoconductor with the projected light pattern. This depletes the charge where the photoconductor is struck by light, and leaves a charge pattern in areas not struck by light (i.e., corresponding to the typed characters in the above example).
The latent image is developed in well known commercial devices by a fine black powder of thermoplastic material called toner, which is attracted to the photoconductor only in its charged areas to establish a visible or toned image. In some processes the photoconductor is consumed in generating the copy in the sense that it remains as the final support. In these processes the toned image is permanently fixed to the photoconductor itself, such as by heat fusing. In other processes the photoconductor is re-usable, and the toned image is transferred to another support, such as paper, to which it permanently is fixed.
Reference is made to U.S. Pat. No. 3,744,900 issued July 10, 1973 in the name of Jorgen Reesen, and to copending U.S. application Ser. No. 481,436, filed on June 20, 1974 in the name of William E. Hunt, et al., now U.S. Pat. No. 3,914,047, patented Oct. 21, 1975 for additional background. To facilitate this description, further discussion will assume processes, similar to those disclosed in said patent and application, which are of the transfer type and include an endless, flexible photoconductor of sufficient length to accommodate several latent or developed images simultaneously.
It should be understood that the image frames on the photoconductor are located with "unused" regions between said frames, i.e., the interframe areas, and with "unused" regions between the frames and the edges of the charge band, i.e., the edge areas. These interframe and edge areas are significant in the context of the present application because they are charged but not exposed to light reflected from the original document. Unless discharged in some manner, they attract and hold toner to the detriment of the machine's operation and its production of acceptable copies. Of course the toner in areas surrounding an image frame would not be transferred to copy paper perfectly registered with the frame, and the excess toner can be cleaned from the photoconductor before the involved section is recycled for its next exposure, but the following problems are some that still remain and have proved difficult to solve. (1) dark edges on the copy paper result from even slight misregister between the paper and the image frame, (2) excessive toner usage requires repeated shut down of the machine for refilling the toner supply, and (3) undesirable burdens are placed on the cleaning station and its power requirements in disposing of the excess toner. To further aggravate the matter, recently designed machines have improved capabilities for developing (toning) large solid areas, including the unused regions lying outside the exposure frames. In these machines the unused toner, discarded through a cleaning cycle, can exceed that which is used on the copies, thereby increasing the cost of running the machine by a significant amount per copy.
Techniques have been devised for avoiding the above noted problems, including, for example; electrical grounding, reverse charging, or special illumination, all of which are designed to remove the unwanted charge in the interframe or edge areas before development. A common technique is to provide an erase lamp capable of discharging the surface of the photoconductor outside of the image frames.
Turning now to a brief consideration of specific disclosures of interest, U.S. Pat. No. 3,792,913 issued to L. A. Simmons on Feb. 19, 1974, U.K. Pat. No. 1,292,571, published on Oct. 11, 1972 in the name of Fuji Foto Film Co., and U.K. Pat. No. 1,332,266 published on Oct. 3, 1973 in the name of Kabishiki Kaisha Ricoh, all disclose apparatus for earsing the charge in the edge areas of the photoconductor prior to development. The apparatus of U.S. Pat. No. 3,792,913 and U.K. Pat. No. 1,332,266 further provide for adjusting the size of the edge erase in accordance with the size of the copy to be produced.
U.S. Pat. No. 3,784,301, issued to M. Sato on Jan. 8, 1974, discloses the use of shields that protect the image frame while the entire border (interframe and edges) is illuminated. It is apparent that the Sato patent is directed primarily to apparatus in which the photoconductor is motionless during exposure.
U.S. Pat. No. 3,788,737, issued to W. L. Kidd on Jan. 29, 1974, shows a platen cover including a source of illumination for discharging the marginal areas surrounding the image frame. This illuminated cover is said to be especially useful when copying books or other thick originals.
U.S. Pat. No. 3,724,942, issued to D. K. Gibson and F. W. Johnson on Apr. 3, 1973, presents a mechanism including a shutter and mirror system for directing light rays to erase the interframe area.
The above mentioned references are directed to problems similar to those addressed by the present application, except that most relate only to edge erase where the solutions have been somewhat easier to attain than in the interframe areas. Where the interframe problem has been considered the proposed solutions have been complex and less than fully satisfactory from the commercial point of view. Extra lamps, single function shields, special timing and register circuits or additional mechanical apparatus generally have been required to accomplish the desired effect.